Frequency modulation detection



y 1945- E. THOMAS 2,379,764

FREQUENCY MODULATION DETECTION Filed June a, 1942 2 Sheet-Sheet 1 1 A I a o o a 0' .L 8 1 i 2 Use/41,1351? 1 OF T 6 EM I J4 I 7 NOISE/CONTROL 5+ Al '15 J NETWURK Y 10 I Q OSCILLATOR T I I u a 0 0 13 13 1 k 5 F1' F; i F2 8 I -B t E w 5 u I Q I INVENTOR /94/P/PY. 77/0/1448.

ATTORNEY July 3, 1945;-

H. E. THOMAS FREQUENCY MODULATION DETECTION Filed June 6; 1942 2 sheets-sneak 2 (5170/1) mung a0; V7709 xv i. M

If mm mukuow EN ax the demodulator,

Patented July 3, 1945 i A 2,379,764 r pp p 2,379,764

FREQUENCY MODULATION DETECTION a arry E. Thomas, Hadd Radio Corporation of A Delawar onfield, N. J,, ;assignor to merica a corporation of I ApplicationJune '6, 1942, Serial No. r -02m My invention relates to detection of angular velocity-modulated carrier waves, and more particularly to oscillator networks adapted for detection of frequency modulatedWFM) carrier waves.--

One of themain objects of my invention isto provide a demodulator of FM waves; wherein the demodulator comprises an; oscillator network having a tank circuit resonant to the mean, or center, frequency of applied FM wave energy, while inputcircuits are provided which are tuned to frequencies on either side of center frequency and differing from the la er by e ual frequency values.

Anotherobject of the inventionis to provide an oscillator system having but a single oscillator tube; the tube having a pair of resonant'input circults to which FM energyi's applied; and the input circuits being oppositely mistuned relative to mean,or center, frequency. l l

Other objects of my invention are to improve the operation of the aforesaid types ofdemodulators; a noise limiting circuit, quickly responsive to noise pulses, acting to render the oscillator inthe oscillator tank circuit frequency which is the effective for the duration of the pulse.

Still other objects of my invention areto im- "prove FlVL demodulators by providing an oscillator type of FM demodulator; the demodulator 5 being capable of inherent limiting, and also being wadapted to supply considerable gain for audio amplification.

The novel features which I believe to be characterlstic of myinventiontare set forth with particularity in the appended claims; the invention itself, however, as to both method of operation dicated diagrammatically several circuit org-ani zations whereby my invention may be carried intoefi'ect.

In the drawingsz Fig. 1 schematically shows a demodulator according to the invention,

its organization and will "best be understood by reference to the following description taken in. connection with the drawings i n which I have in- 40 ll Claim s. (o1. ass- 27) fier, for example. InFM reception the superheterodyne receiver is universally employed.

Thoseskilled in the art arefully acquainted with the networks commonly employed prior to the limiter stage. Assuming, by way of specific illustration, that the receiver "is to be operatedin the assigned FM band of 42 to 50 megacycles (mc.), those skilled in the art know that the resonant selector circuits will have to be sufficientlybroad to pass the maximum frequency deviations-of the carriergorcenter, frequency Fe of each channel.

In FM broadcasting at present, the channel width is 200 kilocycles (kc) although at theFM transmitter-the carrier is deviated to a permissible maximum of 75 kc.on each side of Fe. Of

course, this invention is not limited to the 42 -50 mc. band, nor is it restricted 10f {75 kcwon each side of center'frequency. Furthermore, the generic term angular velocityto ajdeviation range modulated carrier wave is used in this application to denote that either frequency or phase modulated carrier waves may be received. Assuming, now, that the converted FM Waves of a selected channel have been amplified by one or more'I. Fxamplifiers, the amplified I. Frvoltg age across output circuit I is applied to the input electrodes of limiter tube 2. "Ihelatter acts to minimize amplitudemodulation effects the FM wave energy. The effects maybe caused by external noise impulses, tube noise; round-top reso- Fig. 2 shows the discriminator characteristicof Fig; 3'shows amodification or the invention;

Fig. 4.shows the oscillator characteristic of Fig. 3.

Referrin wherein like now to the accompanying drawings,

reference characters in thedifierent figures designatesimilar circuit elements,in Fig.

nance curvesof the cascaded selector: circuits,

man-made static etc. The limiter stage is well known. Its'input circuit 3 istunedto Fe, asis circuit I. latter can be chosen from a as for example 4.3 mc.

l The circuits 1 and 3 will have apass band of 'atleastjlfiq lrcgxand preferably wider,so asto pass efiiciently'all the frequency deviations): The a limiter has a grid circuit network 4-5 whose time constant isshort. The constants of the limiter tube are so chosen thattheinput-outpu! characteristicrises linearly up to a desired input aniplitude; thereafter'it flattens out. Hencafany carrier amplitude increases,representative of undesired amplitude variation, are automatically minimized. During operation there will be a voltage developed across grid resistor l inresponse'to grid current flow through the latter.

If a sharp noise pulse is applied tothe receiver, a

"cuit of anintermediate frequency (I: re amplidirect current voltage is quickly developed across resistor 4 due to the aforesaid grid current flow.

This voltage is used to control the detection of FM wave energy, as will be later explained.

Fe is the operating IL F5 value, and the i range of 2*to 2Qmc.,

oscillator.

' correspond The limited FM energy output of tube 2 is applied to a novel type of demodulator circuit. The latter essentially comprises a pair of oscillator tubes 6 and 1. The control grid 8 of tube 6 is coupled to the limiter plate by coupling condenser 9. The control grid H] of oscillator l is coupled to the limiter plate by coupling condenser ll. Each cathode of the oscillator tubes is at ground potential. The grid of each oscillator tube is connected to ground by a respective grid leak resistor.

A direct current source has its positive terminal B+ (the source being omitted from the drawings) connected to the plate of the limiter tube. The resonant circuit designated F1 has the high potential side thereof connected to the plate side of the coupling condenser 9; the low potential side of circuit F1 is connected .to the 13+ terminal. Similarly, the resonant circuit designated F2 has its high potential side connected in parallel to the limiter plate. The B+ terminal is connected to the low potential side of circuitFz. 3 w

Oscillator tube 6 has its platel2 connected through a blocking condenser 12 to the'coil of a resonant tank circuit designated F0. The resonant'circuit F1 is reactively coupled to the tank circuit of oscillator tube 6. The plate [3 of oscillator tube 1 is connected by blocking condenser 13 to the coil of its tank circuit also designated Fe. 'Here, again, the circuit F2 is reactively coupled to the tank circuit of oscillator tube ,1. 'The symbols Fe applied to the plate resonant circuits of oscillator tubes 6 and 1 signify that they are each tuned to the center, or carrier, frequency of the FM waves. The tank circuits Fe are returned to ground through the B+ terminal.

The symbol F1 applied to the resonant grid circuit of tube 6 indicates that said circuit is tuned to a frequency on one side of ,Fc. The symbol 2 applied to the resonant grid circuit of tube 1 signifies that the said grid circuit is tuned to a frequency on the opposite side of Fe. In Fig. 2 there are shown the respective fifrequency-oscillator output characteristics A and B of each It will be seen that oscillator tube 6 has its peakvalue of output at F1; oscillator 1 has its peakoutput value at F2. The cross-over point of the two characteristics A and B is located. at Ft. It is desirable to locate F and F2 at equal frequency distances from F further, it is preferred to have F1 and F2 beyond the maximum frequency swing. of the carrier. The limits offrequency swing are indicated by the vertical dotted lines on either side of Fe.

The FM signal energy is applied to the grid circuits of oscillator tube 6 and l; regenerative feedback is provided in each case by coupling between each resonant plate circuit and its grid circuit.

Normally, and in the absence of FM waves, both oscillators oscillate at Fe. A the applied FM wave energy deviates in frequency from F the oscillatory output of each oscillator tube will vary as indicated in Fig. 2. Each oscillator is readily able to follow the applied FM wave swings if the instantaneous frequency val- .uesffall between Fe and the frequency of the particular grid circuit. Hence, the plate circuits of the oscillators have currents of variable amplitude flowing therein; furthermore, these currents to the modulation signal variations applied to the carrier at the-FM transmitter. v

To derive the modulation signal energy from the variable-amplitude oscillatory energy, the

plates l2 and i3 are connected toopposite ends of primary winding l4 of transformer l5. In other words, the plate circuits of the oscillators are in push-pull as regards the audio components thereof. If the modulation signals are in the audio range, each plate is connected to its end of the transformer winding through a respective radio frequency choke 16. The midpoint of coil I4 is connected to the positive terminal of a direct current source. Only the audio signal variations pass to the following audio network. Each of the oscillators functions inherently to perform a certain degree of limiting. Accordingly, the limitingdue to tube 2 is augmented.

sister 4 is applied'over lead la-l8 and I8--l8 to the control grids 8 and ID of the respective os-v cillator tubes. Thus, should a sudden pulse of noise be applied. to the receiving system, then the voltage across resistor 4 will be applied quickly to theoscillator grids; The noise control voltage could be sufficiently negative to bias each oscillator tube tocut-off. As soon as the noise pulse disappears, the bias voltage quickly leaks off the oscillator grids and the demodulator is again normally operative. Where the noise pulses occur rapidly, the effectis to produce rapid f aps in demodulation. If the gaps are not too rapid they willnot seriously affect the audio output waveform. Network 4-5 should haveas rapid a time constant as possible. It should allow the noise control voltage to act as quickly as possible in blocking the oscillator, and so reduce any residual noise that passes the limiter. j Discharge of condenser 5 should be rapid so that re turn of the oscillator grid voltage to normal is rapid. i 1

In Fig. 3 I have shown a simplification of the system. Here," a single oscillator tube 20 replaces the two oscillator tubes 6. and 'l of the system of Fig. l. The plate 2i is connected to the high potential side of tank circuit Fe; that is, a single tank circuit replaces the two tank circuits of Fig. 1. The resonant grid circuits F1 and F2 are now arranged in series in the plate circuit of an I. F. amplifier 22. The oscillator grid 23 is connected by the. blocking condenser 24 to the high potential side of circuit F1. The grid leak resistor 25 is connected from the oscillator gridto ground. The B+ terminal of the direct current energizing sourceis connected to feed the I. F. amplifier and oscillator tube plates through the respective coils of the resonant circuits. Each ofcircuits F1 and F2 are reactively coupledto the tank circuit Fe. ,The normal oscillation frequency in.the absence of FM waves is at Fe, and is of low amplitude, v j

The amplifierv 22 may bea limiter. -On the other hand, a special diod rectifier 40 maybe usedv to provide the .quickracting noise control voltage. The diode is coupled, as at 3 3,.to the amplifier input. The lead 52-feedsthe noise control voltage to the control grid 23 of the oscillator. Since the oscillator may be operated at, h h levels, considerable conversion ain of I. F. to audio may be realized with very small l inputs to the prior amplifier, or limiter; As stated before, there is secured augmented limiting action due to theinherent limiting action ofthe oscillator. It

is also, pointed out that the sensitivity of disgranted June 21, 1933, may be employed? "This network comprises the diode rectifiers H and 51 in opposition. The diode, anodes" are connected to opposite sidesof the input circuit 40'. The midpoint of coil 41 is connected bYblOCkillg condenser 42 to the plate side of resonant primary circuit-F6. j The series-arranged load resistors 60iand 6|,

connected betweenthe diode cathodesjhavethe ,junctiontherebf connected to'the midpoint'of coil H through choke coil lDMThe lower end of resistor 61, is grounded, while the audio voltage is taken off from the upperfend of resistor 69.

Reactive coupling; shown by the dotted bracket,

ondary circuit 40'. v v i v "The curve C'inFig. lshowsthe frequencyoscillator outpu characteristic of thefloscillator network in the system of Fig. "3. From the curve exists between the primary circuit 3-252" and ee.

it will be seen that the strength of oscillation increases with departure in either directionfrom oscillator which is locked ,in synchronism with incoming FM waves from amplifier 22. Afcoupling between Fe andAO' produces quadrature phase voltage in circuit "40'. Thexmidpointof coil 4| connected through condenser *42 to the of a discriminator-rectifiernetwork of well-knoii n v the aforesaid center frequency,

push-pull output circuit connecting saidoutput w modea demodulator comreceiveiya network providingfreque lated carrier wave energy,

i prising apair of electrondischargeftubesjeach tube having an electrode andfan output electrode, separate resonant networks coupling said source to respective in-put electrodes of said tubes, said resonant networks "bei gb liositely mistuned by equalfrequency amountsrelative to the centerfrequency of the frequency modulated wave ener y,

coupling each ofthe aforesaid resonant circuits and a respective output; electrode of each t'ube, i

said resonant tank circuits each being tuned to electrodes.

3. In a frequency modulated carrierwave refceiver, a network providing frequency modulated carrier wave energy, a demodulator comprising eachtube ha.v ing an input electrodeand an outputelectrode,

a'pair ofelectron discharge tubes,

separate resonant networks couplingsaid source to respective input electrodes of said tubes, said resonant networks jbeing oppositely mistuned fby high potential side of circuit Fe so that these circuits together .with opposed diodes 50- 51 and associated elements constitute a conventional discriminator. 1'

However, the slope of the discriminator characteristicis increasedaccording to myinvention,

by having th oscillatorvoltage increase as the frequency departs from Fe, instead of decreasing as .does 'the voltage on the primary of theusual discriminator of the aforesaid SeeIey "type. Hence, the usual S-shapedcurve of the discriminator is made more steep due to the actionof-tlie oscillator on primary circuit Fe.

While I have indicated and described several systems .for carrying my invention into'eifect, it willbe apparent .to oneskilled inthe art'that my invention is by nomeans limited to th particular organizations shown and described, but that many modifications may be made Without depart ing from the scope of my invention, as set forth in the appended claims.

What Lelalm is:

1. In combination with a source of angular velocity-modulated carrier waves, a demodulator which comprises an oscillatory network, saidnetwork comprising a pair of resonant circuits, one

ditionalmeanaresponsiVe-to undesirednoise impulses having an amplitude levelfiabove a redeof said resonant circuits being tuned to a frequency located a predetermined frequency amount on one side of the mean frequency of said nection to said electron discharge system for pre- 1 venting operation of the latter for the duration of the impulses.

2. In a frequency modulated carrier wave equal frequency amounts relative toth e center frequency of the frequency modulated wave energy, a resonant tank circuit reactively coupling each of the aforesaid resonant circuits and a respective output electro said center frequency, acommon push-pull cutput circuit connecting said output electrodesfandadtermined value, for rendering said tubesineffective for the duration of the noise impulses.

e 4. Ademodulator for frequency modulated carrier wave energy which comprises an electron discharge tube having an input electrode and an output electrode, a pair of resonant input circuits coupled to said input electrode, said resonant "input circuits being oppositely mistuned by"equal frequency amounts with respect to the center frequency of applied frequency modulationcarriejr waveenergy, a resonant tank circuit providing: a regenerative feedback coupling between said output electrode and said pair of resonantinput circuits, said tank circuit being tuned to said center frequency, and a discriminator-rectifier having an input circuit coupled to said tank circuit.

5. A demodulator for frequency modulated carrier wave energy comprising an electron discharge tube having an input electrode and an output electrode, a pair of resonant input circuits cou-f 55 pled to said input electrode, said resonant input circuits being oppositely mistuned by equal frequency amounts with respect to the center frequency of applied frequency modulation carrier wave energy, a resonant tank circuit providing a regenerative feedback coupling between said outi put electrode and said pair of resonant input circuits, said tank circuit being tuned to said center frequency, means for converting modulated carrier voltage across the tank circuit into modulation signal voltage, and additional means, responsive to input energy above a predetermined amplitude, for automatically rendering said tube ineffective.

6. In a combination with a source of i angular velocity-modulated carrierwaves, a demodulator therefor which comprises a pairof resonant circuits, one of said resonant circuits. being tuned to a frequency located a predetermined frequency amount on one side of the mean frequency of said. waves, the second circuit being tuned to a frea resonant tank circuitreactively and a common e de of each tube, saidres- 1}, if onant tank circuits each being tuned to the aforequency located on the opposite side of said mean frequency at an equal frequency spacing value, means forapplyingsaid wave energy to both of said resonant circuits, an electron discharge tube system having an output circuit across which is developed modulation voltage, means connected to said applying means, responsive to a sudden noise impulse, having a control connection to said tube system for prevention operation of the latter for the duration of the impulse, and means for feeding back to said two resonant circuits oscillatory energy of said mean frequency. c

'7. In a frequency modulated carrier wave receiver, a demodulator comprising a pair of electron discharge tubes, each tube having at least an input electrode and an output electrode, a pair of resonant circuits oppositely mistuned-by equal frequency amounts relative to the center frequency of the frequency modulated wave energy, a separate resonant tank circuit reactively coupling each of the aforesaid resonant circuits and a respective output electrode of each tube, said resonant tankcircuits each being tuned to'the aforesaid center'frequency, and a common push"- pull output circuit connecting said output electrodes.

8, In a frequency modulated carrier wave receiver, a network providing frequency modulated carrier wave energy, ,a demodulator comprising a pair of electron discharge tubes, each tube having an input electrode and an output electrode, separate resonant networks coupling said source to respective input electrodes of said tubes, said resonant circuits being oppositely mistuned by equal frequency amounts relative to the center frequency of the frequency modulated wave energy, a separate resonant tank circuit reactively coupling each of the aforesaid resonant circuits and a respective output electrode of each tube, said resonant tank circuits each being tuned to the aforesaid center frequency, a common push-pull audio output circuit connecting said output electrodes, and additional means, responsive to undesired noise impulses having an amplitude levelabove a predetermined value, for rendering said tubes ineffective for the duration of the noise impulses.

9. A demodulator for frequency modulated carderiving modulation signals from oscillations to vary in rier Wave energy which comprises an electron discharge tube having an input electrodeand an output electrode, a pair of. resonant input circuits coupled to said input electrode, said resonant input circuits being oppositely mistuned by equal frequency amounts with respect to the center frequency of applied frequencymodul'ation car rier Wave energy, a resonant tank circuit providing an oscillation-producin feedback coupling between said output electrode and said pair of resonant input circuits, said tank circuit being tuned to said center frequency, and means for 10. A demodulator for frequency modulated carrier wave energy which comprises an electrondischarge tube having aninput electrode-andan output electrode, a pair of resonant input circuits coupled to said input electrode, saidresonantinput circuits being oppositely mistuned by "equal frequency amounts with respect to thecenter frequency of applied frequency modulation car rier wave energy, a resonant tank circuit providing a regenerative feedback coupling between said output electrode and said pair of resonant input circuits, said tank circuit being tuned to said center frequency, means to derive modulation signal voltage from the energy developed across 'thetank circuit, and additional means, responsive to input energy above a predeterminedamplitude,

for automatically rendering saidtube ineffective.

11. In combination with a source of frequency modulated waves, a detector provided with a frequency discriminatory network input, said network having an input circuit tuned to ,a predetermined frequency, said Waves having a mean frequency substantially equal to said predetermined frequency, an oscillator tube provided with input and output electrodes and including said input circuit between its input and output electrodes as a regenerative coupling thereby normally to provide oscillations of said predetermined frequency, said oscillator tube bein auxiliary to said detector,-means couplin said source to said tuned input circuit thereby to cause said frequency in synchronism with the frequency variation of said waves.

HARRY E. THOMAS.

the tank circuit. 

